The development of Ni base superalloys, particularly those intended to form single crystal articles for use at elevated temperatures such as 1800.degree. F. and above, has identified certain phases or conditions which can form in the alloy structure and which can be detrimental to the useful life of the article. One such detrimental condition has been referred to as a Secondary Reaction Zone (SRZ). This formation consists essentially of an orthorombic "P" phase which is a Type II topographically close-packed (TCP) phase, and a gamma phase, both dispersed in a gamma prime matrix. The TCP phase and the gamma phase form needle-like structures within the blocky gamma prime matrix.
Problems associated with the formation of SRZ after exposure of an alloy at temperatures of at least about 1800.degree. F. have been discussed in U.S. Pat. No. 5,270,123--Walston et al, patented on Dec. 14, 1993; and the formation of the TCP phase has been identified in U.S. Pat. No. 5,151,249--Austin et al, patented on Sep. 29, 1992. Also, the TCP phase is addressed in U.S. Pat. No. 5,173,255--Ross et al, patented Dec. 22, 1992, relating to an alloy including about 1.5-4 wt. % Re. The disclosures of each of these patents, assigned to the assignee of this invention, hereby are incorporated herein by reference.
The above incorporated Walston et al patent discloses that the potential for the formation of SRZ within the cast structure of a Ni base superalloy can be predicted and controlled by alloy content, particularly the element Re at greater than about the 4 wt. % shown in the above incorporated Ross et al patent, in the presence of other alloying elements. In solidification of Ni base superalloy single crystal castings, elements segregate either to the dendrites or interdendritic regions. Re segregates heavily to the dendrites and thus internal SRZ typically is observed in dendrites of such alloys including Re.
Some Ni base superalloys, such as those which are identified in the Walston et al patent, have been developed to avoid and rarely do form internal SRZ. However, under certain conditions a small amount of SRZ has been observed in an article surface portion adjacent or under an environmental coating, such as an aluminide or a Pt--Al diffusion coating or an overlay coating at least partially diffused into a surface portion; for example, the well known MCrAlY type of coatings in which "M" generally is Ni, Co, or their combination. Such a condition can have a detrimental effect on mechanical properties of an article including SRZ in such surface portion.